Researchers studying the teeth of two 1,500-year-old German victims of the Justinian plague have uncovered tiny bits of DNA that link the ancient disease to the deadly plague of just a few centuries ago.

By reconstructing the genome of the ancient bacteria, the scientists found that the Justinian Plague, which killed millions in the Byzantine Empire in the 6th Century, was caused by a strain of the same pathogen that led to the Black Death, which killed an estimated 50 million Europeans in just a few years in the 14th century. The researchers published their findings in the journal Lancet Infectious Diseases.

“What this shows is that the plague jumped into humans on several different occasions and has gone on a rampage,” Tom Gilbert, a professor at the Natural History Museum of Denmark, told the AP. “That shows the jump is not that difficult to make and wasn’t a wild fluke.” His point? The plague could happen again.

But it’s not as ominous as it sounds. Each year, there are several thousand cases of plague in humans across the world, usually spread by fleas on rodents that carry the bacteria. Scientists warn that will continue as human populations continue to infringe on rodent territory. While still deadly, modern epidemics have been much less devastating than ones hundreds of years ago.